1. Field of the Invention
This invention relates to a tri-tinted lens for spectacles, and more particularly concerns a tri-colored lens for special-purpose sunglasses for use during flying a jet aircraft. These tri-colored lens reduce glare from high illumination in a far field of vision when viewing in an intermediate or near fields of vision. The tri-colored lens greatly improves the pilot's ability to see the instrument panel and make viewing between highly contrasting fields of vision less stressful to the eyes.
2. Description of the Related Art
In the field of optical wear, a wide variety of sunglasses and goggles have been developed over the years for various purposes, depending upon the specific function of the sunglasses or the activity involved while wearing the sunglasses. Optical wear has been created to reduce glare from point sources of light, such as from oncoming cars or from direct sunlight. Optical wear has also been created to reduce the glare from broad regions of high illumination, such as office lighting, sky glare, and ground glare. Optical wear also has been developed for directing sight into specific fields of vision. This type of eyewear enables the viewer to assume a correct posture or position for carrying out a physical activity; the great majority of the prior art in this application has been in developing golfing glasses. Finally, optical wear has been designed to reduce visual stress in contrasting lighting situations and to enhance feature extraction in different fields of vision, these types of optical wear can be useful during physical activities where it is desirable to see a specific object against a background, such as golfing, fishing, hunting or other outdoor recreational activities.
One area that prior art has not addressed is the situation encountered by pilots of jet aircraft, which are flown typically at altitudes between 35,000 and 45,000 feet. At these heights during the day, the illumination from outside the cockpit is especially high. The high illumination comes from scattered light from both the sky and clouds. Indeed, when flying over clouds, pilots typically experience illumination values around 9,000 lux even when they are not looking in the direction of the sun. By contrast, under these external lighting conditions, the illumination of the instrument panel can be typically around 30 lux. In this case, the ratio between the outside illumination to that of the instrument panel is 300 to 1. Making transitions between these two contrasting fields of illumination can be stressful to the eyes.
Of the variations of sunglasses disclosed in prior art, none has satisfactorily solved the problem of balancing the illumination between outside and inside the cockpit—a problem that pilots flying at altitude encounter. Ordinary dark sunglasses do not solve this problem; while they do reduce the outside illumination to a comfortable level for the pilot, they can render the instrument panel too dark for the pilot to discern readings on the gauges. On the other hand, vignetted sunglass lenses assist in the overall balancing of illumination outside the cockpit with that of the instrument panel, but they permit too much light to enter a pilot's eyes, thus causing glare when the pilot views the instruments.
The limitations of prior art described above highlight the need for sunglasses that specifically address the two and often related problems that many jet pilots routinely encounter when flying at altitude: that is, (1) the contrast in illumination causes stress on the pilot's eyes resulting from the eyes having to adjust back and forth between the high illumination from outside the cockpit and the low illumination from the instrument panel; and (2) the glare caused by the high illumination can degrade the ability of the pilot to adequately see the instruments. Moreover, these two problems are exacerbated as the pilot ages.
To fully understand the problem the specially tinted lens solve, it is helpful to define the difference between the horizon and the horizontal plane in which the aircraft is flying. The horizon is plane tangent to the earth's surface at an observer's position and the horizontal plane is perpendicular to the vertical axis to the earth. Between 35,000 ft and 45,000 ft the horizon is approximately 3.3° to 3.8° below the horizontal plane. Consequently, whenever the aircraft is above a uniform layer of cloud that extends to the horizon, the illumination from that cloud layer originates from below the horizontal plane.
The illumination that a pilot experiences when flying at these high altitudes can be significantly different from that experienced by a person on the ground. For example, when pilots fly above the clouds, the illumination experienced when they look horizontally and below is typically greater then when they look above the horizontal plane. This high illumination at and below the horizontal plane poses a unique problem for pilots when they look at the instruments, in that the high illumination from outside the cockpit can be relatively close to their intermediate field of vision of the instruments. The reverse tends to be the case for a person on the ground, where the illumination generally increases when looking upwards.
The specially tinted lens, when adapted to sunglasses, enable a pilot to easily view outside the cockpit into high illumination and simultaneously view the instrument panel in low illumination without the eyes having to compensate for the previously described problems of adjustment and glare. The specially tinted lens reduces visual stress to the eyes by greatly reducing the difference in illumination as received by the eyes between lighting outside the cockpit and the instrument panel. In addition, this reduction in light received by the eyes from outside the cockpit greatly reduces glare that can impair the pilot's vision of the instrument panel.
The specially tinted lens addresses the specific and unique condition present in the cockpit of a jet aircraft where the instrument panel and nose of the aircraft block the high illumination from outside the cockpit from entering the intermediate section of the lens.